Lead

Flux fumes

These are the bigger danger. Get a fume extractor. Do not breathe them in.

Solder

Use leaded solder, even on lead free devices. It melts at much lower temperatures, is safer (as lead isn’t the primary risk), and is easier to see bad joints.

Use 60/40 leaded. 63/37 eutectic is OK but I find that it is harder to get good joints when tacking wires, and it makes it harder to detect when parts are about to come loose during desoldering.

Nearly all solder is “flux cored” i.e. there is flux in the middle. Most of the difference you will see in solder is down to the flux. This part from Loctite has a relatively active flux that is good for rework:

Do not breathe flux fumes. Use a fume extractor if possible (or breathe at the right times if you’re feeling brave!)

Tips

If using Metcal, you can’t change the temperature without changing the tip.

Metcal tips come in three temperatures called F (normal), T (temperature sensitive i.e. low temp) and C (ceramic i.e. high temp). Generally you only need F due to the way they regulate power.

If the Metcal display is showing high power usage (bar graph full all the time) and you can’t melt solder, you either don’t have enough contact or the tip is too small. Wet the tip or try a bigger tip.

The Metcal stand has magnets in it to quench the field in it, so it cools down. You don’t need to turn it on and off all of the time.

If using an adjustable iron, 370degC is the normal temperature. A bit lower for SMT, a bit higher for bigger stuff.

As you get better, generally you can tweak the temperature up a bit to speed up.

For normal joints

Place tip against part to solder, wait a fraction of a second, introduce solder, add enough solder, remove tip and solder.

Leaded joints should be bright, shiny and concave.

For tacking/bodging

Sometimes it is easier to place solder onto the test pad or component and then place the wire.

Sometimes it is easier to hold the wire in place and “load” the tip with solder first.

Experiment! Get some cheap boards and see what you can do.

This isn’t a cheap board, it’s the Instrument Cluster (IC) board from a Tesla model S that we’re working on. It does give you an idea of how much soldering can be required as part of an IoT test though.

Desoldering

You will be surprised how much heat components can take before they are damaged.

Decide if you want to remount the component or re-use the rest of the device. We will take more care if we do.

Soldering iron + desoldering tools (either wick or desoldering pump). This is good for through-hole components, and generally for 8 and 16 PIN SOIC. Don’t be scared to add more solder to make lead-free into leaded, and to retain a bit of heat.

Soldering iron + ChipQuick (solder that contains Bismuth and melts at 138degC). This is good for NAND flash and similar. Gets the melting point very low and lets you work the device with an iron.

Hot air tool. Best for BGA. Preheat the board with ~180degC air keeping the tool moving. Change to 400degC and move it around the component you want off. Airflow should not be so fast as to blow small components off.

Toaster oven/heatgun. Get the whole board hot and smack it on something hard to get all components off.

Retronix – are great at rework and device removal. If it is fiddly or damaging the device will be expensive, get the professionals to do it. Better to spend a few £ getting rework done than risk trashing a board or component. http://www.retronix.com/.

After chips are desoldered, they often need cleaning. Use extra flux and solder wick. Careful not to damage pins or landing pads.

Reballing

Some BGA devices will need reballing to be read in a socket.

For very small devices such as EMMC, this can be done with a lot of flux and solder – it will make the pads proud of the surface enough for a socket to work.

If you need to remount the part, it will certainly need reballing.

Currently I use Retronix to do this as it’s often cheaper to outsource this type of work.

Bodges

We often find ourselves wanting to monitor or inject lots of signals. You learn a lot of bodges!

30AWG Kynar wire is your friend here. There is lots in the lab. It is hard to strip – use a proper tool. I always strip both ends as there is nothing more frustrating than stripping one end, soldering it, and then ripping the pad off the board soldering the other end.

Some people swear by PTFE insulated magnet wire. This should be “stripped” by the heat of the soldering iron… I just can’t get comfy with it.

Test pads can often be soldered to. Dab small amount of solder onto the test pad, then join the wire.

It is often stronger to find a through-via and solder the wire to that. 30AWG should fit through most vias.

Empty through-hole connectors can be filled with pin headers. We have all of the common pitches available. At a push, you can just use the pins without the plastic carrier.

SMT pads for pin headers can be soldered to with normal pin headers – just use a bit more solder!

It is easy to tack wires onto 2.54mm pitch connectors/SMD. 1mm is fine for most people. 0.5mm is a bit of a push (but possible!).

You can also abrade the solder resist (green stuff) off the board and then solder to that. I just use a scalpel to remove it.

Use hot glue for strain relief. Do not cover the whole board as you will then find you need to access a trace under the glue. At a push, it can be removed with a heat-gun and patience.